1. Field of the Invention
The present invention relates to an improved process for making sodium phosphates (e.g. disodium phosphate, trisodium phosphate and sodium tripolyphosphate). More specifically, it relates to the recovery of phosphate (P.sub.2 O.sub.5) values from the solid precipitate (muds) formed by the neutralization of impure wet process phosphoric acid with a sodium compound.
2. Brief Description of Prior Art
One method for making phosphoric acid today is by the so-called wet process. By this process phosphate rock is first digested with a mineral acid, usually sulfuric acid, phosphoric acid, or combinations of the two, which results in the precipitation of calcium values from the rock and the release of an impure phosphoric acid solution. This impure acid solution is relatively high in impurities, including silica, iron and aluminum. The silica is usually then removed as a sodium silicofluoride precipitate by addition of a small amount of sodium hydroxide or sodium carbonate.
In the course of making sodium phosphates, the impure, partially neutralized phosphoric acid (after removal of the calcium and silica) is further neutralized, normally by addition of more sodium hydroxide or carbonate, to form a monosodium phosphate solution. During this neutralization step, a major portion (at least about 50% by weight) of both the undesirable iron and aluminum impurities are precipitated from the solution. While the resulting monosodium phosphate solution is substantially pure, the iron- and aluminum-containing precipitates have therein a relatively large amount of phosphate values, thus reducing the overall yield of the process. Typically, about 5% to about 15% by weight of the total P.sub.2 O.sub.5 values of the phosphate rock are present in these precipitates or muds.
The recovery of such P.sub.2 O.sub.5 values from these muds is not easy. Extraction of the P.sub.2 O.sub.5 values with an aqueous sodium carbonate or sodium hydroxide solution results in the co-extraction of the aluminum values (the iron values are left in the muds). Until the present invention, the co-extraction of the aluminum was undesirable since it had to ultimately be either re-precipitated as a phosphate, thereby reducing the overall efficiency of the extraction process, or had to be contained in the final product, thereby reducing product purity and, possibly, the performance of the product.
Accordingly, there is a need to improve this extraction of muds to prevent the carry through of aluminum values with the phosphate values. The present invention offers a solution to this need.